The aim of this research is to determine the effects of visual deprivation on visual development by studying children treated for dense, central cataracts. An unusually large sample of these children is being treated at The Hospital for Sick Children by surgically removing the cataract and fitting the resulting aphakic eye with a contact lens. A major objective is to compare the effects of monocular versus binocular deprivation in order to elucidate the contribution of competition between the eyes versus deprivation per se. Based on preliminary results, we predict that the visual abilities following monocular deprivation, which involves both deprivation and competition between the eyes, will follow a pattern similar to that shown by strabismic amblyopes, by normal human infants, and by monocularly deprived animals. We predict that the visual abilities following binocular deprivation, which involves primarily deprivation per se, will be less severe and follow a pattern similar to that shown by anisometropic amblyopes and by binocularly deprived animals. Reducing the competition in unilateral cases by extensive patching of the fellow eye should increase the similarity between unilateral and bilateral cases. We predict, as well, that recovery from deprivation may occur only as a trade- off between eyes. We propose to study (a) contrast sensitivity, (b) color vision, (c) vernier acuity, (d) grating acuity, (e) letter acuity, (f) the symmetry of optokinetic nystagmus, (g) peripheral vision, (h) binocular vision and (i) motion perception. In each case we will (1) compare the effects of monocular and binocular deprivation, and in monocular cases, the effect of patching; (20 relate the findings to the normal pattern of development, to deficits in strabismic versus anisometropic amblyopia, and/or to deficits in monocularly versus binocularly deprived animals; and (3) study the influence of the timing and duration of deprivation. Comparisons among visual functions will indicate whether, as in animals, there are multiple sensitive periods. Such comparisons may clarify the mechanisms by which deprivation affects visual development and provide clues to underlying neural deficits. Our results will delineate the sensitive periods for different visual functions and hence the speed with which ophthalmologists need to treat patients with different etiologies (e.g., unilateral congenital versus developmental cataract). They will indicate the value of patching in unilateral cases following deprivation beginning at different ages and lasting for different durations. They will also allow clinicians to make more accurate statements about the prognosis in individual cases. Finally, we propose new tests of letter discrimination and vernier acuity which may prove to be valuable additions to existing clinical tools for assessing the vision of infants.